This invention relates to methods and apparatus for reducing pulsations in the flow produced by a multiple piston assembly pump of the kind which provides an overlap between the flow output of the piston assemblies during a portion of the strokes of the piston assemblies.
This invention relates particularly to a pump used for liquid chromatography.
The present invention has particular application to a dual piston pump of the kind disclosed in U.S. Pat. No. 4,352,636 issued Oct. 5, 1982 and assigned to the same assignee as the present application; and this U.S. Pat. No. 4,352,636 is hereby incorporated by reference in this application.
Pulseless flow of the liquid supplied to a column for liquid chromatography is desirable to permit the column to achieve greater resolution of peaks. Pulseless flow also permits the detector to operate at a more sensitive range (to detect smaller peaks), and pulseless flow is less restrictive on an integrator used in liquid chromatography apparatus.
A constant flow over a long term is necessary for repeatability of the retention times of the chromatogram peaks and is a factor in the area of the peak.
As disclosed in U.S. Pat. No. 4,352,636, the control for a dual piston pump of this kind has included two control loops--a first fast responding control loop for controlling the pressure that the pump is pumping and a second slow responding control loop for producing a slow responding pressure set point for monitoring average pump speed.
The dual control loop theory works with any multiple piston pump, but there can be a problem in achieving proper control during the time when the pump is changing over from one piston pumping to the other piston pumping. During the piston changeover, the motor speed is not a good indicator of flow. The duration of the changeover period can vary with differences in the compressibility of different solvents and with errors and irregularities in the external system (such as, for example, variability in closing time of check valves, small imperfections in the cams driving the pistons, etc.). At this point in the cycle of operation, there can be a sudden pump speed change, it is desirable to run the pump at a substantially constant pump output pressure until the pump speed returns to the normal speed existing prior to the sudden speed change. If the response rate of the slow responding control loop is made slow enough to hold the pressure constant during this changeover period, then the response rate of this slow responding control loop must also be slow in responding both to the system solvent changes and to initial start-up, and this is undesirable.
It is a primary object of the present invention to detect a sudden pump speed change during the time of piston changeover and to run the pump in a constant pressure mode until the speed returns to normal.
In a specific embodiment of the present invention this object is accomplished by forcing the input of an integrator located in the slow responding control loop to zero in response to and during the duration of the time involved between the detection of the sudden pump speed change and the return to the normal pump speed.
By forcing the integrator to hold the pressure constant during the changeover period, the second slow control loop can be provided with a much faster response time than would be the case than if the integrator did not hold the pressure constant at this point in the cycle. This permits the second slow responding control loop to provide better response to system solvent changes and initial start-up, and this is an important advantage in the overall pump control.